The invention relates to the field of trunked mobile telephone systems. A trunked system is a radio network which by means of shared use of radio channels offers the services of a company-specific radio network to several organizations. Trunked systems comprise control exchanges, base stations and mobile telephones. Trunked systems typically have a cellular structure, whereby each cell has one or more radio channels which are further divided into time slots, of which one or more are used as signalling channels, while the others are used as traffic channels. The signalling channels are used for call establishment, registrations, etc. The traffic channels are used for transmitting speech and for circuit mode data connections.
The invention is suitable for use particularly in mobile telephone systems with a digital radio path. One digital mobile telephone system is described in standard ETS 300 392-2:1994, Radio Equipment and Systems (RES); Trans-European Trunked Radio (TETRA); Voice plus Data (V+D), Part 2: Air interface, ETSI, 625 pages.
The invention is intended to be applied particularly to PMR mobile telephone or mobile communication systems (PMR=Private Mobile Radio). PMR systems are typically used by companies or authority organizations. Mobile telephone systems can be based, for example, on FDMA or TDMA technology. The invention is intended to be applied especially in TDMA systems (TDMA=Time Division Multiple Access).
When channels are allocated between base stations and subscriber stations, there are typically a limited number of channels available, since there are only a small number of radio frequencies and/or time slots. In modern mobile communication systems, it is often important to allow a mobile station to have a signalling connection (Control Channel) with the base station at the same time that the mobile station in question has a telecommunication connection, i.e. a speech or data connection, with the base station. The signalling connection is needed, for instance, for transmitting to the mobile station information on other, beginning or ongoing calls, short data messages or status messages. There are prior art solutions in which a radio unit involved in a call has at suitable intervals moved for short periods to other frequencies to listen to them, i.e. monitored or scanned the desired adjacent channels. However, such a solution does not enable reliable reception of short data messages and status messages at a mobile station. Furthermore, switching the receiver of a radio unit to another frequency, i.e. for scanning, requires guard times, during which the radio unit cannot be used for communication.
In FDMA systems (FDMA=Frequency Division Multiple Access), the possibility of transmitting signalling messages or other kinds of data during calls or data calls is called in-band signalling. In this system, part of the transmission capacity of a channel is typically used during a call for signalling, which displaces the actual speech or circuit mode data.
In addition, it is known to fixedly reserve part of the traffic capacity allocated to one user for signalling. This is called out-band (channel-associated) signalling.
The problem with the prior art solutions is that the control channel capacity according to the protocol of the prior art TETRA mobile telephone system is extremely limited during speech items in individual and group calls. During a speech item, only a slow control channel is available, once a second. Certain services, and particularly implementations intended for PMR networks used by authorities, require higher signalling capacity during speech items.